1. Field of the Invention
This invention relates generally to waterborne coating compositions containing 1,3,5-triazine carbamate crosslinkers, methods of preparing such coatings, water-dispersible compositions suitable for use in preparing such coatings, and articles coated therewith.
2. Description of Related Art
Various derivatives of amino-1,3,5-triazines are described in the literature for use in a wide variety of fields. Certain of these derivatives, such as alkoxymethyl derivatives of melamine and guanamines, are useful as crosslinkers or reactive modifiers in curable compositions which contain resins having active hydrogen groups. While alkoxymethylated melamines and guanamines provide excellent results in a number of aspects, they also have the disadvantage of releasing formaldehyde as a volatile byproduct under curing conditions. It has long been a desire of industry to find acceptable alternative crosslinkers which do not emit formaldehyde upon cure.
One such alternative group of crosslinkers-which show great promise are the 1,3,5-triazine carbamates which are disclosed in one or more of commonly owned U.S. Pat. No. 4,939,213, U.S. Pat. No. 5,084,541, U.S. Pat. No. 5,288,865, EP-A-0604922 (corresponding to U.S. application Ser. No. 07/998,313 (filed Dec. 29, 1992)), EP-A-0624577 (corresponding to U.S. application Ser. No. 08/061,905 (filed May 14, 1993)), U.S. application Ser. No. 08/138,581 (filed Oct. 15, 1993), U.S. application Ser. No. 08/239,009 (filed May 6, 1994), U.S. application Ser. No. 08/286,835 (filed Aug. 5, 1994) and U.S. application Ser. No. 08/324,549 (filed Oct. 18, 1994), all of which are hereby incorporated by reference herein for all purposes as if fully set forth. The 1,3,5-triazine carbamates disclosed in these references are believed to react with active hydrogen group-containing resins in a manner similar to blocked isocyanates, and have been found to be particularly useful as crosslinkers in coating compositions based upon hydroxy-functional resins, with the cured coatings possessing a wide range of desirable properties. Among the most desirable properties are lowered (or zero) formaldehyde emission upon cure, and improved acid etch resistance in the cured films.
In a number of the previously incorporated references, it is disclosed that curable compositions containing polyfunctional active hydrogen-containing polymers and 1,3,5-triazine carbamate crosslinkers can be adapted for use in organic solvent-based and powder coating compositions. For example, in previously incorporated EP-A-0604922 (U.S. application Ser. No. 07/998,313) are disclosed organic solvent-based coatings prepared from tin- and acid-catalyzed systems using 1,3,5-triazine carbamate crosslinkers. Also disclosed therein are the use of the 1,3,5-triazine carbamates as co-crosslinkers in combination with conventional amino resin crosslinkers to improve the acid etch resistance, and lower formaldehyde emission, of traditional organic solvent-based amino resin crosslinked systems.
Despite the excellent films which can be achieved with these systems, the coatings industry is under great pressure to reduce the environmentally undesirable emission of volatile organic compounds (VOC), which includes the organic solvent media. One means of doing so, of course, would be to exchange some or all of the liquid organic solvent medium with water. Although it is generally disclosed that curable compositions containing the 1,3,5-triazine carbamate crosslinkers may be adapted for use in water-based systems, the switch from organic solvents to water is neither a simple nor straightforward matter, particularly in the case of 1,3,5-triazine carbamates which are substantially hydrophobic and substantially non-dispersible in water.
A specific disclosure of the use of 1,3,5-triazine carbamates in waterborne systems can be found in previously incorporated U.S. Pat. No. 4,939,213 and U.S. Pat. No. 5,084,541, in which is described (example 9) an aqueous electrocoating bath (containing about 10% solids) prepared by combining a typical cationic electrodeposition resin, tris-(2-ethylhexyloxycarbonylamino)-1,3,5-triazine (as crosslinker), lactic acid (neutralizing agent), dibutyltindilaurate (catalyst) and water. Such electrodeposition coatings are generally cationic, low solids systems which are not suitable for application by conventional spray technology.
There is, thus, clearly a need for low VOC curable coating compositions employing 1,3,5-triazine carbamate crosslinkers, and particularly those which are adapted to be applied by conventional spray technology such as utilized in automotive original equipment manufacture (OEM) and general industry applications.